The invention is based on a guide-vane system for an axial-flow exhaust-gas turbine connected upstream of the guide-vane system.
Such a guide-vane system is described, for example, in EP 0 131 719 B1, in particular in an embodiment according to FIGS. 3 to 5. In this guide-vane system, an adjusting ring constantly acted upon by an adjusting force in one direction is provided concentrically to the turbine impeller, this adjusting ring being arranged radially on the outside in the flow passage. Each of the guide vanes is pivotable about a radially oriented axis located in the region of the leading vane edge. The trailing edge of each guide vane bears against an end face of the adjusting ring. By adjusting the adjusting ring parallel to the turbine axis, the guide vanes can be pivoted between two end positions, and a uniform inflow to the turbine impeller can thus be achieved. Depending on the intensity of the exhaust-gas flow, the guide vanes are automatically adjusted in the most favorable angular position in each case.
DE-C1-36 23 001 and DE-C2-41 00 224 each disclose guide-vane systems in which a vane shank directed radially outward through a casing wall from the flow passage is attached to each of the guide vanes. With its part directed outward, the vane shank is mounted so as to be rotatable about a pivot axis. In the guide-vane system disclosed in DE-C2-41 00 224, the guide vanes are adjusted by means of a toothed segment which is fastened to each shank end and in each case interacts with a toothed rim arranged outside the flow passage and rotatable about the turbine axis.
In the guide-vane system in DE-C1-36 23 001, the guide vanes are adjusted by an adjusting ring, arranged outside the flow passage and rotatable about the turbine axis, and by an adjusting lever which transmits a torque from the adjusting ring to the vane shank of each guide vane. The adjusting ring is mounted on connecting straps of a fastening ring in a rotatable manner by means of rolling elements, the fastening ring being fastened to the turbine casing. On account of the high forces which have to be transmitted during the adjustment of the adjusting ring, this design involves the risk of the components jamming and of self-locking occurring.
The invention as defined in the patent claims achieves the object of specifying a guide-vane system of the type mentioned at the beginning, which, despite a simple construction, is characterized by high reliability even under heavy operating conditions.
According to the invention, an adjusting ring, which is part of a guide-vane system of the type mentioned at the beginning, is mounted with its outer surface on rolling elements which are each designed as one-armed levers which are mounted so as to be pivotable on a component of a casing wall, this component being designed as a supporting ring. Self-locking of the guide-vane system is countered by this mounting. The guide-vane system is therefore characterized by high operational reliability and a long service life and can also be acted upon by adjusting force in an extremely simple manner.
It is especially advantageous to design those ends of the lever arms which are remote from the supporting ring in such a way that in each case a guide lug fastened to the adjusting ring can engage in the lever arm. It is likewise very favorable to provide guide grooves in these ends, the guide grooves in each case guiding a section of the adjusting ring. The guide grooves each have a groove root which rests on the outer surface of the adjusting ring and is curved in the opposite direction to the outer surface and on which a section of the outside of the ring can roll.
Undesirably high bearing forces on the vane shank are avoided if that part of the vane shank which is directed outward is mounted at two bearing points which are radially offset from one another. The bearing points are advantageously arranged in that component of the casing wall which is designed as supporting ring. The supporting ring and the guide vanes, guided at the bearing points in a pivotable manner, can then be fitted in the exhaust-gas turbine in an extremely simple manner or, if desired, can easily be removed again in a simple manner. Low bearing forces at the two bearing points are definitely ensured if the bearing points are situated far apart. The supporting ring therefore has a large overall height in the radial direction.
In order to protect the supporting ring from large temperature differences and thus high thermal stresses, which may impair the reliability and service life of the guide-vane system, the supporting ring is screened on its inside from the flow passage carrying hot exhaust gases by a relieving ring defining the flow passage on the outside. Although the supporting ring comes into contact with ambient air on its outside, high thermal stresses can be greatly reduced in this way.
In order to ensure a simple and reliable adjustment of the guide vanes, the adjusting levers provided in the guide-vane system according to the invention should in each case be connected at one of their ends to the vane shank and should be guided at their other end in a groove of the adjusting ring. This can advantageously be achieved with simple means by the adjusting lever being slipped onto the vane shank, and by a spacer supported on the adjusting ring being fastened to the adjusting lever. The spacer then ensures that, irrespective of the position of the adjusting lever, there is always reliable and effective transmission of force from the adjusting ring to the vane shank, and that self-locking of the guide-vane system according to the invention is definitely avoided. If there is no spacer, the adjusting lever, during operation of the guide-vane system, could otherwise be displaced away from the vane shank in the direction of the adjusting ring on account of external forces, such as engine vibrations for example. During the adjustment of the guide-vane system, the adjusting ring would then have to push back the adjusting lever against the high frictional forces of the slip-on connection, which could lead to jamming and to self-locking of the guide-vane system. Reliable and effective transmission of force is also ensured when the adjusting lever is slipped onto the vane shank and when a locking element engaging behind the vane shank is fastened to the adjusting lever. This transmission of force is distinguished by an especially high degree of reliability, since, unlike the spacer, the locking element is only loaded with relatively low deformation forces during operation of the guide-vane system. In each case, however, it is highly advisable to design the adjusting lever like a spherical head at its end guided in the groove of the adjusting ring, since force or torque is then transmitted virtually free from backlash from the adjusting ring to the vane shank. For space-saving reasons, it is advisable to connect each of the adjusting levers to the vane shank between the two bearing points.
If a cooling passage which can be connected to a coolant source is formed in the supporting ring and is directed via the casing leadthroughs of the vane shanks into the flow passage, the service life and operational reliability of the guide-vane system according to the invention are additionally increased. At the same time, the escape of hot and aggressive exhaust gas from the flow passage can thereby be avoided with great certainty.
It is advisable to attach the vane shank to the guide vane in such a way that the pivot axis runs between the leading edge of the guide vane and a line of the guide vane which connects the vane pressure points. The vanes are then always loaded with a defined torque produced by the exhaust-gas flow. Under certain operating conditions, vibration of the guide vane, which otherwise possibly develops, can thus definitely be avoided or at least substantially suppressed. If the guide-vane system fails, the guide vanes are then opened by the exhaust-gas flow, as a result of which the speed of the exhaust-gas turbine is reduced and overspeeding is avoided.